1. Field of Invention
The present invention relates to digital-to-analog converters. More particularly, the present invention relates to a digital-to-analog converter having multiple DAC sections to process different voltage levels according to an input word.
2. Description of Related Art
Conventional resistor string (R-string) digital-to-analog converters (DAC) use a string of resistors to provide various voltages levels and a selector including select lines corresponding to each voltage level. The conventional R-string DAC requires a total of 2N select lines to convert N bits of data. In an age where chip size continues to shrink, using a large number of select lines such as 2N select lines is not area efficient and may waste much chip space.
In U.S. Ser. No. 11/563,321, a dual output DAC was disclosed to reduce the number of select line from 2N to 2N-1, which saves a significant amount of area. Please refer to FIG. 1, a block diagram of the dual output DAC. The dual output DAC includes an R-string section 102, a DAC section 104, a multiplexer section 106, and an operational amplifier 108. An input word may be divided into a least significant bit (LSB) and the remaining bits. The DAC section 104 selects two voltage levels from the R-string section 102 according to the remaining bits of the input word. The multiplexer 106 then selects one of the voltage levels according to the LSB, so that the operational amplifier 108 may average the first voltage of the two voltage levels with the selected voltage level as the final output of the DAC. Therefore the dual output DAC requires only 2N-1 select lines to accomplish converting an N-bit input word.
However, the operational amplifier 108 is limited in its voltage input range. Thus, the gamma curve produced by the DAC may be distorted when the operational amplifier 108 attempts to process very large voltage differences. For example, when the input word selects voltage levels near the rail voltages, distortion may occur to produce a non-linear output gamma curve.
For the forgoing reasons, there is a need for a new type of DAC, where distortion caused by the operational amplifier is accounted for in the design to provide a linear gamma curve.